1. Field of the Invention
The invention relates to apparatus for detecting particles flowing in a duct and a method of detecting contaminants on the particle detector.
2. Description of the Prior Art
The performance of a triboelectric particle detector may be impaired if particles from the flow are allowed to build up on its surface. One way to avoid the build up of particles is to clean the probe on a regular basis. However, cleaning involves taking the particle detector off-line and may be time-consuming and require the disassembly of complex equipment. It is therefore desirable to monitor the level of contamination of the detector.
U.S. Pat. No. 5,287,061 describes an on-line method of monitoring a detector for contamination. In the method described in that patent, a detector in the form of a probe is triboelectrically charged by particles in a flow. The signal generated by that charging passes through monitor circuitry that includes an amplifier, the gain of which is dependent upon the impedance of the probe. Under normal operating conditions, the impedance of the probe is substantially infinite. However, when the probe becomes contaminated the impedance may drop significantly, and that drop results in a change in the gain of the amplifier. In order to detect the occurrence of any such change in gain, a second signal of known amplitude is passed through the amplifier. A change in the magnitude of the amplified second signal indicates that the gain of the amplifier has changed and that cleaning of the probe may be required.
A low probe impedance can also cause the gain of the amplifier to rise with respect to the amplifier""s internal noise; i.e., the gain seen by the input signal is unchanged, but the gain seen by the internal noise is increased. That causes an increase in the background noise level of the instrument, and that increase may be enough to mask low-level signals.
However, a finite probe impedance does not always cause a problem in a particle detection system. In particular, in a detector in which A.C. coupling is used, such as those described in British Patent No. 2266772 and British Patent No. 2277154, a finite probe impedance usually only has a significant effect if it is so low as to saturate the amplifier, or if it varies in magnitude with time. Each of those effects would produce an error signal with an A.C. component which would be detected by the A.C. monitoring apparatus. In most other circumstances a finite impedance would usually only result in a D.C. signal caused by the offset-error voltage of the amplifier. The A.C. coupling circuitry would filter out such a D.C. signal.
A more significant problem for both A.C.- and D.C.-coupled detectors occurs when contaminants on the detector act as a signal source. Particles flowing past the detector can interact with the contaminants and cause the generation of A.C. or D.C. error signals. The impedance-dependent contamination monitoring technique described above would not detect signal-generating contamination unless the contamination also resulted in a low probe impedance.
It is an object of the invention to provide a method and apparatus for detecting particles flowing in a duct and for detecting contamination which is acting as a signal on the particle detector.
According to the invention, there is provided a particle detector for detecting particles flowing in a duct, comprising: a first triboelectric probe for detecting the particles and a second triboelectric probe for detecting a signal generated by contaminants on the particle detector.
The first probe may be used for monitoring the particle flow in the usual way. The presence of signal-generating contaminants on the particle detector will result in an error signal on the first probe. The error signal will also be detected on the second probe because the second probe is located between the first probe and the structure. Therefore, the presence of a signal on the second probe will indicate that the signal appearing on the first probe includes an error signal. The degree to which contamination is a problem may be determined by considering the ratio of the magnitude of the signal generated on the second probe to the magnitude of the signal generated on the first probe. If this ratio exceeds a certain pre-defined limit then cleaning of the detector will be required.
Advantageously, the second probe substantially surrounds the first probe. Each of the first and second probes may take any of several forms; for example, at least one of the probes may be in the form of a rod (which may be of circular cross-section), a ring or a stud.
The particle detector may be so arranged that, if the detector is mounted on a duct, both probes will project into a flow of particles flowing in the duct. Alternatively, the particle detector may be so arranged that, if the detector is mounted on a duct, neither probe will project into a flow of particles flowing in the duct. Preferably, the particle detector is so arranged that, if the detector is mounted on a duct, the second probe does not project into a flow of particles flowing in the duct. If both probes project into the particle flow, the second probe should be so arranged that it generates a significantly smaller signal attributable to the particle flow than the signal generated by the first probe. The ratio of the signals from the two probes would remain approximately constant when the detector was not contaminated, but a ratio significantly different from that constant value would indicate that the detector was contaminated. Even with the second probe out of the air stream, it is possible that the second probe will detect a low-level signal attributable to the flow of particles.
Also according to the invention there is provided apparatus for detecting particles flowing in a duct, the apparatus comprising: a particle detector, which includes a first probe and a second probe, the first and second probes being so arranged that, if the particle detector is mounted on a structure, the second probe will be positioned between the first probe and the structure, and the first and second probes will be substantially electrically isolated from each other and from the structure; means for monitoring a signal generated on the first probe; and means for monitoring a signal generated on the second probe by contaminants on the detector.
The means for monitoring the signal generated on the second probe may be electronic circuitry substantially identical in design to that used to monitor the signal generated on the first probe. The means for monitoring the signal generated on the second probe may be substantially independent of the means for monitoring the signal generated on the first probe. Alternatively, the means for monitoring the signal generated on the second probe may utilise the means for monitoring the signal generated on the first probe.
The monitoring means may include means to monitor A.C. components of the signals and may furthermore include filtering circuitry which removes D.C. components from the signals. Alternatively, the monitoring means may include means to monitor D.C. components of the signals.
Also according to the invention, there is provided a particle-detecting installation, including a duct together with a particle detector as described above. Also according to the invention, there is provided a particle-detecting installation, including an apparatus for detecting particles as described above. Both probes may project into the flow of particles. Alternatively, neither probe projects into the flow of particles, Preferably, the second probe does not project into the flow of particles. The duct may be a stack.
Also according to the invention, there is provided a method of detecting contaminants on a particle detector, which involves the use of a particle detector as described above. Also according to the invention, there is provided a method of detecting contaminants on a particle detector, which involves the use of an apparatus for detecting particles as described above. Also according to the invention, there is provided a method of detecting contaminants on a particle detector, which involves the use of a particle detecting installation, as described above.
The present invention also provides a method of detecting contaminants on a particle detector comprising: providing a particle detector, which comprises a first triboelectric probe for detecting particles flowing in a duct and a second triboelectric probe; monitoring a signal generated on the first probe; and monitoring a signal generated on the second probe by the contaminants.
The signal from the second probe may be monitored continuously via monitoring circuitry that is substantially independent of monitoring circuitry used to monitor the signal from the first probe. Alternatively, the signal from the second probe may be monitored intermittently via monitoring circuitry used to monitor the signal from the first probe, with the monitoring circuitry being switched between the two probes. The latter method requires less circuitry but the former enables monitoring to be carried out more frequently without taking the first probe off-line.
Preferably, A.C. components of the signals generated on the first and second probes are monitored. Alternatively, the D.C. components of the signals generated on the first and second probes may be monitored. Alternatively, the signals including both A.C. and D.C. components may be monitored.
The present invention also provides a method of detecting contaminants on a particle detector, the method comprising: providing a particle detector, which is mounted on a structure and includes a first triboelectric probe and a second triboelectric probe, which is positioned between the first probe and the structure, the first and second probes being substantially electrically isolated from each other and from the structure; and monitoring a signal generated on the second probe by the contaminants.